This invention relates generally to an improved slot closure for the stator of a dynamoelectric machine, and in particular it relates to an improved slot closure having a transverse spring which forms the closure.
In larger dynamoelectric machines there are slots formed in a stator core which comprises a laminated stack of iron sheets. The slots have winding conductors installed in them and the conductors must be held firmly in place against mechanical, thermal and electromagnetic forces which tend to cause movement. The slots are usually formed with a form of groove or notch, sometimes in the shape of a dove-tail, adjacent the mouth of the slot and a member having a mating configuration is pressed into the slot with the mating edges extending into the groove to close the slot mouth or slot opening. Shims are frequently used between the slot closing member and the conductors as a height adjustment to bring the slot contents up to a desired height.
One of the earlier slot closures is described in Canadian Pat. No. 679,228--Jaun et al issued Feb. 4, 1964. In this patent a slot bridging member of soft iron is inserted into the slot opening with the edges of the slot bridging member extending into notches on either side of the slot opening, and then the slot bridging member is distorted or deformed to cause the edges to press more firmly into the notches and hold the slot bridging member more firmly in place. This slot bridging member of soft iron was intended primarily to regularize the flux pattern and it appears that the retaining of the conductors was not a major consideration.
At the present time, conductors used in larger dynamoelectric machines usually have a jacket formed of a thermosetting resinous material impregnating a porous material, and this is cured to a hard state forming an insulating jacket around the conductor. In addition there may be portions of the jacket coated with a partially conducting elastomer to reduce the possibility of voids developing between the conductors and the slot walls. The elimination or reduction of voids tends to eliminate or reduce corona problems. Conductor arrangements of this type are described in Canadian Pat. No. 1,016,586--Lonseth et al, issued Aug. 30, 1977 to Canadian General Electric Company Limited.
When conductor arrangements as described in the aforementioned Lonseth et al patent are used, they are firmly wedged into their slots to restrain them against movement. However, with time, the possibility of decreased restraining forces has been a problem. Repeated thermal cycling may result in some "bedding in" at the high contact points and some creep or flowing of the materials within the slot. These factors may result in a gradual decrease in height of the slot contents, and if the slot closure system is rigid then the force placed on the slot contents by the slot closure will decrease as the height of the slot contents decreases. The machines presently used may carry considerable current in the conductors and the electromagnetic forces can be quite large and of a vibrational nature, for example, at two times operating frequency. It is very important that the restraining forces should not decrease below a certain level. If the restraining forces decrease enough to permit any vibration, the continuous vibrational forces will eventually cause a major problem.
In order that the restraining forces be maintained, it is known to introduce some form of spring between the slot closure member and the conductors. These springs have been made in various forms and kinds. When a spring is used between a slot closure member and the conductors, there is usually some arrangement to compress or deform the spring so that the spring will in fact exert a force against the closure member on one side and the conductor on the other side.
One prior art arrangement is described in Canadian Pat. No. 980,398--Brown et al, issued Dec. 23, 1975, where two wedge members having complementary tapers are placed in the slot in an overlapping position so that as the wedge members are pressed or advanced towards one another, their overall thickness increases. A spring member in the form of an extended rectangle is also placed in the slot adjacent the wedges. This spring member has transversely extending ripples or deformations to provide a spring action. The two wedge members are pressed together to compress the spring and this exerts the desired force between the slot closure member and the conductors.
Another prior art arrangement is described in Swiss Pat. No. 580,351--Lottanti et al, dated Sept. 30, 1976. This patent shows a slot closure member whose edges engage opposing dove-tail notches in the slot walls. The slot closure member has a longitudinally extending, transversely curved, inner surface. A spring support member has an oppositely curved outer surface. A flat strip of spring material is placed between the mating curved surfaces (not extending into the notches) and is deformed into a similarly curved configuration to provide a force between the slot closure member and the conductors. This arrangement provides for spring forces extending substantially continuously along the length of the slot.
Another arrangement which provides a substantially continuous spring force along the length of the slot, i.e., a line loaded spring, is described in Canadian Pat. No. 1,095,108--Ferguson, issued Feb. 3, 1981, and assigned to Canadian General Electric Company Limited. In this arrangement a transversely curved, longitudinally extending spring member is compressed or flattened by cooperating wedges to provide a spring force between the slot closure member and the conductors.
In the prior art slot closure arrangements which use a spring member, a preferred material for the spring member is often a composite such as a cured epoxy resin with strands of glass fibers embedded therein. Composite materials such as these tend to lose certain of their mechanical properties at elevated temperatures, that is, when they are deflected or deformed and subjected to an elevated temperature they tend to take a permanent set. The tendency increases with temperature. Therefore, it is desirable to keep the temperature of a spring member of composite material as low as possible.
To summarize, dynamoelectric machines of current design may have conductors carrying large currents, and in addition to providing adequate retaining force which would accommodate any shrinkage, it is desirable that the operating temperature of the spring member be as low as possible.